1. Field of the Invention
The present invention relates to a thermal sensor and a heating structure used in the thermal sensor for detecting flow rate, acceleration, pressure, etc., and particularly to a thermal sensor and a heating structure used in the thermal sensor for use in applications such as thermal flow rate sensors in which extremely small changes in resistance are amplified and appear as output fluctuations.
2. Description of the Related Art
In a first conventional thermal flow rate sensor heating structure, a supporting film made of Si3N4 or SiO2 is formed by coating over an entire surface of a silicon substrate, a heating resistor made of platinum is formed on the supporting film, and a protecting film made of Si3N4 or SiO2 is further formed on the supporting film so as to cover the heating resistor. Then, a cavity is formed by removing the silicon substrate under the region where the heating resistor is disposed from a rear surface side until the supporting film is reached. Thus, a heating structure having a diaphragm construction composed of the supporting film, the heating resistor, and the protecting film is formed above the cavity. (Patent Literature 1, for example.)
In a second conventional thermal flow rate sensor heating structure, a supporting film and a protecting film constituting a diaphragm portion are formed such that a slight tensile stress arises relative to a silicon substrate. (Patent Literature 2, for example.)
Patent Literature 1: Japanese Patent Laid-Open No. HEI 4-2967 (Gazette, FIG. 1)
Patent Literature 2: Japanese Patent Laid-Open No HEI 11-194043 (Gazette, FIG. 2; paragraph 0012)
In the first conventional heating structure, if a material such as SiO2, which tends to generate compressive stress, is used in the supporting film and the protecting film constituting the diaphragm portion, the diaphragm portion deforms due to thermal expansion when the heating resistor is generating heat. The larger the diaphragm portion, the greater the deformation of the diaphragm portion. Thus, heat dissipation characteristics from the diaphragm portion change, giving rise to irregularities in measurements.
In the second conventional heating structure, on the other hand, because the supporting film and the protecting film constituting the diaphragm portion are formed such that a slight tensile stress arises relative to the silicon substrate, deformation of the diaphragm portion due to the heat generated by the heating resistor can be suppressed. However, no mention has been made concerning a specific numerical value for the slight tensile stress, or the relationship between the size of the diaphragm portion and the tensile stress.
In view of these conditions, the present applicants have selected silicon nitride films for the supporting film and the protecting film constituting the diaphragm portion, and have found that silicon nitride (Si3N4) films having tensile stress can be stably formed by adjusting flow rates of argon, nitrogen, etc, and gas pressure inside a vacuum chamber, etc., in sputtering apparatuses, and by adjusting the flow ratio of material gases such as silanes, ammonia, etc., in plasma chemical vapor deposition (CVD) apparatuses, thereby leading to the invention of the present invention. It has been found that, depending on the manufacturing apparatus, it is necessary to use films rich in silicon (Si) relative to the stoichiometric composition ratio between Si and nitrogen (N) in an Si3N4 film in order to obtain silicon nitride films having tensile stress.
However, as described below, one problem has been that reliable output characteristics cannot be achieved in heating structures constituted by a heating resistor composed of a platinum film held between a supporting film and a protecting film formed using these silicon-rich silicon nitride films.
When consideration is given to fouling due to adsorption of moisture, it is preferable for the temperature conditions in which this heating structure is operated to be such that heat is generated at a temperature 100 degrees Celsius (100xc2x0 C.) or more higher than the ambient atmospheric temperature, but under such temperature conditions, if heating is performed by continuously passing an electric current through the heating resistor of a heating structure constituted by a heating resistor composed of a platinum film held between a supporting film and a protecting film formed using these silicon-rich silicon nitride films, a phenomenon is observed in which the resistance value of the heating resistor after the heating due to the continuous passage of electric current changes relative to the resistance value before heating due to the continuous passage of electric current. The fluctuations in output characteristics occur as a result of the fluctuations in the resistance value of the heating resistor after the heating due to the continuous passage of electric current.
Furthermore, even if the supporting film and the protecting film constituting the heating structure have a slight tensile stress, if the tensile stress is too small when a rectangular diaphragm portion has a large size in which the length of the short sides thereof is 1 mm or more, the diaphragm portion expands when the heating resistor is generating heat, giving rise to bending. This bending, in other words deformation, of the diaphragm portion changes the heat dissipation characteristics of the diaphragm portion, giving rise to irregularities in output.
The present invention aims to solve the above problems and an object of the present invention is to provide a thermal sensor in which the reliability of output characteristics is high by reducing surplus silicon (Si) in a silicon nitride film to suppress silicidation of a heating resistor, which is a factor contributing to resistance value increases, and prescribing a magnitude for a tensile stress in the silicon nitride film to suppress deformation of a diaphragm portion occurring when the heating resistor is generating heat.
In order to achieve the above object, according to one aspect of the present invention, there is provided a thermal sensor including a flat base material through which an opening is disposed; and a heating structure having a diaphragm construction constructed by forming a supporting film on a first surface of the base material so as to cover the opening, forming a heating resistor composed of a platinum film on a portion of the supporting film above the opening, and forming a protecting film on the heating resistor. The thermal sensor is characterized in that at least one film of the supporting film and the protecting film is constituted by a silicon nitride film having an index of refraction of less than 2.25.
Therefore, silicidation of the platinum film resulting from heat generated by the heating resistor is suppressed, thereby providing a thermal sensor in which the reliability of output characteristics is high.
According to another aspect of the present invention, there is provided a thermal sensor including a flat base material through which an opening is disposed; and a heating structure having a diaphragm construction constructed by forming a supporting film on a first surface of the base material so as to cover the opening, forming a heating resistor composed of a platinum film on a portion of the supporting film above the opening, and forming a first protecting film on the supporting film so as to cover the heating resistor. The thermal sensor is characterized in that the supporting film and the first protecting film are each constituted by a silicon nitride film and the heating structure is constructed so as to have a tensile stress of greater than or equal to 50 MPa and less than or equal to 250 MPa.
Therefore, deformation of the diaphragm portion of the heating structure resulting from heat generated by the heating resistor is suppressed, thereby providing a thermal sensor in which the reliability of output characteristics is high.
According to yet another aspect of the present invention, there is provided a thermal sensor including a flat base material through which an opening is disposed; and a heating structure having a diaphragm construction constructed by forming a supporting film on a first surface of the base material so as to cover the opening, forming a heating resistor composed of a platinum film on a portion of the supporting film above the opening, forming a first protecting film on the heating resistor, forming an intermediate film on the supporting film so as to cover the first protecting film, and forming a second protecting film on the intermediate film. The thermal sensor is characterized in that the supporting film, the first protecting film, and the second protecting film are each constituted by a silicon nitride film, the heating structure is constructed so as to have a tensile stress of greater than or equal to 50 MPa and less than or equal to 250 MPa, and the intermediate film is formed to a film thickness of less than or equal to one tenth ({fraction (1/10)}) of a sum of a film thickness of the supporting film and a film thickness of the second protecting film.
Therefore, deformation of the diaphragm construction of the heating structure resulting from heat generated by the heating resistor is suppressed and tolerance against deformation by external forces is increased, thereby providing a thermal sensor in which the reliability of output characteristics is high and durability is superior.